Why is rainwater less salty than water commonly used for irrigation?

Rainwater is less salty than water commonly used for irrigation because of how each is formed and collected. Rainwater is essentially distilled water, created through the water cycle when sunlight evaporates water from oceans or lakes, leaving salts behind and resulting in condensation that falls as relatively pure, fresh water.12

In contrast, water used for irrigation is typically sourced from runoff or groundwater. As this water moves across the landscape or percolates through the soil, it physically erodes rocks and chemically dissolves the minerals and salts it encounters. By the time this water reaches rivers, streams, or aquifers to be collected for agricultural use, it has accumulated a significant concentration of these dissolved substances. When this irrigation water is applied to crops, the water evaporates or is transpired by plants, leaving those collected salts behind in the soil.1234